Kinetics of trichloroethene (TCE) reduction by zero-valent iron: effect of medium composition

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1 Groundwater Quality: Natural and Enhanced Restoration of Groundwater Pollution (Proceedings ofthe Groundwater Quality 2001 Conference held at Sheffield, UK, June 2001). IAHS Publ. no Kinetics of trichloroethene (TCE) reduction by zero-valent iron: effect of medium composition J. DRIES, L. BASTIAENS, D. SPRINGAEL, L. DIELS Vito, Flemish Institute for Technological Research, Department of Environmental Boeretang 200, B-2400 Mol, Belgium Technology, S. N. AGATHOS Unit of Bioengineering, Catholic University of Louvain, Place Croix du Sud 2/19, B-l348 Louvain-la-Neuve, Belgium Abstract In a series of batch experiments, we investigated the effect of various medium composition parameters on the kinetics of reductive dechlorination of trichloroethene (TCE) by zero-valent iron filings. Calculated TCE half-lives in dilute simulated groundwater were not affected by increasing sulphate concentrations, increasing solution ionic strengths or increasing initial ph values. The presence of the organic co-pollutants tetrachloroethene, cu-dichloroethene or toluene, at low loading rates, did not interfere with TCE degradation. TCE reduction kinetics were slightly affected by the presence of hydrogen sulphide at a high initial concentration, which was supposedly caused by the formation of iron sulphide precipitates. Increasing concentrations of nitrate on the other hand dramatically impacted TCE removal by zero-valent iron. The reduction of nitrate, to equivalent amounts of ammonium, apparently passivated the iron surface. Finally, the reduction of TCE by zero-valent iron was retarded to some extent in a landfill leachate contaminated groundwater characterized by a high organic carbon content. Key words kinetics; landfill leachate; medium composition; mixed pollution; nitrate; reductive dechlorination; TCE; zero-valent iron INTRODUCTION The efficacy of an in situ zero-valent iron permeable reactive barrier for treating chlorinated organics in a complex matrix, as encountered in landfill leachate polluted aquifers, may depend on the composition of the contaminated groundwater. We therefore investigated, in a series of batch experiments, the effect of various medium composition parameters on the kinetics of reductive dechlorination of the model compound trichloroethene (TCE) by zero-valent iron filings. MATERIALS AND METHODS Set-up of batch test systems Each batch test consisted of a control set (i.e. base medium without iron), a reference set (i.e. medium with iron, no other addition) and a number of treatment sets with different values of the parameter under investigation. Table 1 gives an overview ofthe different medium composition parameters tested. The medium was a simulated ground-

2 398 J. Dries et al. Table 1 Overview of the batch experiments testing effects of various solution composition parameters on TCE reduction kinetics. Test Parameter Test description 1 Nitrate Increasing nitrate concentrations: 0, 10, 100 and 1000 mg 1"' nitrate 2 Sulphate Increasing sulphate concentrations: 0, 100, 250 and 1000 mg l" 1 sulphate 3 Sulphide Increasing sulphide concentrations: 0, 250 and 500 mg 1" 1 hydro; *en sulphide 4 Initial ph Increasing initial ph values: ph 7.7, 8.8 and Ionic strength Increasing ionic strengths: 0.004, and 0.04 M 6 Co-pollutants Presence of co-pollutants: PCE (2 mg 1"') or 1,2-cis-DCE (4 mg T')or toluene (2.5 mg 1"') 7 Landfill leachate Real landfill leachate contaminated groundwater water consisting of NaHC0 3, KHC0 3, CaCl 2.2H 2 0 and MgCl 2.6H 2 0, all at 0.5 mm. A series of amber serum bottles (volume approx. 26 ml) was set up for each parameter investigated. Zero-valent iron filings (-6.28 g), with a specific surface area of m 2 g~', were added without pretreatment to reaction flasks to obtain a 1:4 iron to liquid ratio. The bottles were subsequently filled with TCE contaminated simulated groundwater leaving no headspace, using a separatory funnel. TCE was added from a saturated solution to obtain an initial concentration of approximately 10 mg F '. The bottles were capped with Teflon-faced rubber septa, and incubated on a horizontal rotary mixer (speed -10 rpm) at room temperature. Each batch experiment ran for 15 to 20 days. At regular time intervals starting at day 0, we removed one bottle of each set for analysis. KINETIC MODELLING A pseudo first order model C = Co exp(-fe) was applied to describe the reductive dechlorination of the parent compound TCE by zero-valent iron (Johnson et al, 1996). The first order rate constant k for each reactive set was estimated by linear regression of the logarithmically transformed data vs time. The half-life /j/ 2 of the parent compound was then calculated according to /j/ 2 = (In 2)1 k. ANALYSES The concentration of chlorinated ethenes was determined with an Interscience GC 800Top gas chromatograph connected to a Voyager MS detection system. The concentration of free chloride was quantified colorimetrically using a Skalar segmented flow analyser. Ammonium (ref , 14558), nitrate (ref ) and sulphate (ref ) were quantified colorimetrically using the Spectroquant cell tests from Merck. RESULTS AND DISCUSSION General overview The reference sets in each of the batch tests described in Table 1 were prepared and operated almost identically in all experiments (e.g. temperature, iron loading, mixing

3 Kinetics of TCE reduction by zero-valent iron: effect of medium composition 399 rate, liquid composition, etc.). The average half-life of TCE calculated for the independent reference sets was 15 ± 3 h. The small relative standard deviation of the average reference kinetic parameter (about 18%) suggested that the results obtained for the reference sets were quite reproducible. We therefore compared the TCE half-lives estimated in the different batch tests with this average reference half-life. The results for tests 1 to 3 (Table 1) and tests 4 to 7 are represented in Figs 1 and 2 respectively. 100! reference half-life 95% confidence interval I tf <# / >.c*v ^ fs>> A * <s» KÇ» N 10 Fig. 1 Estimated TCE half-lives in the batch tests with increasing nitrate, sulphate and sulphide concentrations, in comparison to the average TCE half-life in the reference sets (error bars represent the standard error of the linear regression analysis). -reference half-life 30-95% confidence interval J 20 UJ o H 10 0 Fig. 2 Estimated TCE half-lives in the batch tests with increasing ph values, ionic strengths, and in the presence of selected co-pollutants or a landfill leachate contaminated groundwater, in comparison to the average TCE half-life in the reference sets (error bars represent the standard error of the linear regression analysis). The reductive dechlorination kinetics of TCE by zero-valent iron were not affected by increasing sulphate concentrations (Fig. 1) or by increasing ph or ionic strength values (Fig. 2). The presence of the co-pollutants tetrachloroethene (PCE), dichloroethene (DCE) and toluene (TOL) did not significantly influence the TCE half-life (Fig. 2). Increasing concentrations of nitrate, on the other hand, had a dramatic impact on the TCE degradation kinetics (Fig. 1). High sulphide concentrations also seemed to slightly affect the TCE dechlorination. Finally, we observed a moderately negative effect of the landfill leachate contaminated groundwater (Fig. 2). Each of these observations will be discussed in more detail below.

4 400 J. Dries et al. (a) (b) O 0.8 O g- 0.6 o" 4 Z X X \ t(d) X- B x o mg/l nitrate A- 100 X 1000 O 0.8 O = 0.6 X z A" u x x x t(d) X mg/l nitrate A--100 X 1000 Fig. 3 Relative concentration profiles of N0 3 "-N (a) and NH/-N (b) in the batch test with increasing concentration of nitrate. Nitrate effects Nitrate below or at 100 mg 1" ' had no significant effect on the TCE reduction kinetics (Fig. 1). TCE dechlorination in the presence of 1 g T 1 nitrate however was almost five times slower than in the absence of nitrate. Figure 3 shows the fate of nitrate in our zero-valent iron batch systems. Nitrate, especially at low concentrations, was quickly reduced by zero-valent iron to equivalent amounts of ammonia (Fig. 3), as reported earlier (Cheng et al, 1997; Huang et al, 1998; Zawaideh & Zhang, 1998). Schlicker et al. (2000) observed that nitrate reduction effectively competed with TCE dechlorination in zero-valent iron column systems. The authors found that increasing concentrations of the strong oxidant nitrate ( mg T 1 ) significantly delayed the onset of TCE degradation in column systems. Sulphate and sulphide effects Lipczynska-Kochany et al. (1994) investigated the effects of the sulphur compounds sulphate (at 4.8 g 1"') and sulphide (at 1.6 g l" 1 ) on the batch degradation kinetics of carbon tetrachloride (CT) by zero-valent iron powder. The authors observed a slight improvement of the performance of the iron in the presence of sulphate. The degradation rate of CT more than doubled in the presence of sulphide (Lipczynska-Kochany et al, 1994). We found that sulphate did not influence TCE removal kinetics (Fig. 1). In the presence of 0.5 g l" 1 sulphide however, the TCE half-life doubled (Fig. 1). An almost immediate formation of black iron sulphide precipitates was visible in the serum flasks, which may have hindered the reaction of TCE at the iron surface. In contrast, Butler & Hayes (1999, 2000) found that many halogenated aliphatics, including PCE and TCE, were rapidly dehalogenated by iron sulphide in batch tests. The authors used crystalline iron sulphide (i.e. mackinawite), which may react differently than the amorphous precipitates formed in our experiment. ph effects The corrosive oxidation of zero-valent iron is a proton consuming reaction (Matheson & Tratnyek, 1994). Such reactions become thermodynamically less favourable when

5 Kinetics of TCE reduction by zero-valent iron: effect of medium composition 401 the ph increases. As the reduction of chlorinated ethenes depends on the corrosion of iron, a negative trend in the reductive dechlorination kinetics is to be expected with increasing ph. Indirect ph effects, such as iron hydroxide precipitation at high ph, may also influence dechlorination rates by passivation of the reactive surface. Matheson & Tratnyek (1994) found a negative linear relationship (R 2 = -0.92) between the first-order CT degradation rate constant and solution ph ranging from 5 to 10. We did not observe any effect of increasing initial ph values ( ) on TCE degradation kinetics (Fig. 2). However, CT degradation reported by Matheson & Tratnyek (1994) was very fast (zj/ 2 = 15 min) compared to the TCE removal rates found in our experiments. The authors also used a 15-times lower iron surface concentration to treat comparable contaminant concentrations. Under such conditions, it is conceivable that contaminant degradation rates are more sensitive to ph changes. Ionic strength effects A two to ten fold increase in ionic strength of the simulated groundwater, achieved by using more concentrated simulated groundwater with the same relative composition, did not affect the reduction of TCE by zero-valent iron (Fig. 2). The increased amounts of bicarbonate, calcium and magnesium ions in solution may cause calcium carbonate (CaCC>3) and siderite (FeCCb) precipitation on the iron, potentially affecting the reactivity of the surface (Mackenzie et al, 1999). The effects of these precipitates however can probably not be investigated in short-term batch experiments (Farrell et al, 2000). Mixed pollution effects The presence of PCE, DCE and toluene at environmentally relevant concentrations did not influence the reductive dechlorination of TCE by zero-valent iron, when each of these compounds was supplied individually with TCE (Fig. 2). These results accord with Burris et al. (1995) who found that TCE and PCE did not compete for reaction. The authors performed batch experiments with a low contaminant loading of pmol g" 1 iron" 1, comparable to the loading applied in our test (0.3 pmol g" 1 iron" 1 ). Arnold & Roberts (2000) used a somewhat higher specific loading (about 2.5 pmol g" 1 ) for the target compound in a series of competitive batch experiments. The potential competitors, however, were supplied at significantly higher concentrations ( pmol g" 1 iron" 1 ). Substantial inhibitory effects were observed under these circumstances (Arnold & Roberts, 2000). Landfill leachate effect In one of a few studies in this specific field, Schreier & Reinhard (1994) investigated the transformation of PCE by metallic iron in buffered water and two landfill leachate contaminated groundwaters. The authors found that the complex aqueous matrix of the leachate did not affect the PCE dechlorination kinetics. In contrast, we found that the reduction kinetics of spiked TCE were significantly affected (i.e. by a factor 2.5) in a landfill leachate contaminated groundwater compared to the dilute simulated ground-

6 402 J. Dries et al. water reference (Fig. 2). The leachate was characterized by a negligible nitrate content, a relatively high specific conductivity (i.e. approx us cm" us cm" 1 in the simulated groundwater), and a considerable amount of ill-defined organic carbon (i.e. a TOC content of 109 mg l" 1, probably of fulvic-like nature). As a tenfold increase in the solution ionic strength did not affect the TCE degradation (see above and Fig. 2), it is unlikely that the higher conductivity of the leachate caused the observed inhibition. The role of TOC content remains unexplored so far. Tratnyek & Scherer (1998) found that natural organic matter, especially humic acids, negatively impacted CT dechlorination by zero-valent iron. CONCLUSIONS The findings of this study suggest that the composition of the liquid medium in contact with zero-valent iron may influence its reactivity towards chlorinated contaminants in some cases. The most pronounced effect recorded was the impact of increasing concentrations of the strong oxidant nitrate. The moderate but significant inhibition of TCE dechlorination kinetics observed in the complex aqueous matrix of a landfill leachate contaminated groundwater warrants further investigation. Acknowledgements We would like to thank Miranda Maesen for her assistance. This research was financially supported by a Vito PhD grant to J. Dries. REFERENCES Arnold, W. A. & Roberts, A. L. (2000) Pathways and kinetics of chlorinated ethylene and chlorinated acetylene reaction with Fe(0) particles. Environ. Sci. Technol. 34, Burris, D. R., Campbell, T. J. & Manoranjan, V. S. (1995) Sorption of trichlorocthylene and tetrachloroethylene in a batch reactive metallic iron-water system. Environ. Sci. Technol. 29, Butler, E. C. & Hayes, K. F. (1999) Kinetics ofthe transformation of trichloroethylene and tetrachloroethylene by iron sulfide. Environ. Sci. Technol. 33, Butler, E. C. & Hayes, K. F. (2000) Kinetics ofthe transformation of halogenated aliphatic compounds by iron sulfide. Environ. Sci. Techno/. 34, Cheng, F., Muftikian, R., Fernando, Q. & Korte, N. (1997) Reduction of nitrate to ammonia by zero-valent iron. Chemosphere 35, Farrell, J., Kason, M., Melitas, N. & Li, T. (2000) Investigation of the long-term performance of zero-valent iron for reductive dechlorination of trichloroethylene. Environ. Sci. Technol. 34, Huang, C. P., Wang, II. W. & Chiu, P. C. ( 1998) Nitrate reduction by metallic iron. Wat. Res Johnson, T. L., Scherer, M. M. & Tratnyek, P. G. (1996) Kinetics of halogenated organic compound degradation by iron metal. Environ. Sci. Technol. 30, Lipczynska-Kochany, E., Harms, S., Milburn, R., Sprah, G. & Nadarajah, N. ( 1994) Degradation of carbon tetrachloride in the presence of iron and sulphur containing compounds. Chemosphere 29, Mackenzie, P. D., Homey, D. P. & Sivavec, T. M. (1999) Mineral precipitation and porosity losses in granular iron columns../. Hazard. Mai Matheson, L. J. & Tratnyek, P. G. (1994) Reductive dehalogenation of chlorinated methanes by iron metal. Environ. Sci. Technol. 28, Schlicker, O., Erbert, M., Fruth, M., Weidner, M., Wiist, W. & Dahmke, A. (2000) Degradation of TCE with iron: the role of competing chromate and nitrate reduction. Groundwater 38, 403^109. Schreier, C. G. & Reinhard. M. (1994) Transformation of chlorinated organic compounds by iron and manganese powders in buffered water and in landfill leachate. Chemosphere 29, Tratnyek, P. G. & Scherer, M. M. (1998) The effect of natural organic matter on reduction by zero-valent iron. In: Preprints of extended abstracts presented at the 216th ACS National Meeting (Boston, Massachusetts) 38, Zawaideh, L. L. & Zhang, T. C. (1998) The effects of ph and addition of an organic buffer (HEPES) on nitrate transformation in Fe -water systems. Wat. Sci. Technol. 38,